1. Field of the Invention
The present invention relates to an electronic device including an interface such as a USB, for example. Particularly, the present invention relates to an electric power supply section thereof.
2. Description of the Related Art
Hitherto, an electrical device including a USB (Universal Serial Bus) terminal (port), for example, such as a mouse, a keyboard, a CD-ROM, a CD-R/RW, an MO, and a DVD, has been known.
Using the USB interface, not only transmission and reception of data but also an electrical power supply from a host to a device is possible, and such device is called a bus-power device. In the bus-power device, a low power device having consumption current of maximum 100 mA and a high power device having consumption current of maximum 500 mA are defined. In addition, a device including its own power supply means such as an external power source is correspondingly called a self-power device.
As described above, since there is an upper limit of maximum 500 mA to a power supply cable of the USB interface, and since it is necessary to secure electric power independently on a device side by, for example, separately mounting an AC adaptor or a battery when a device whose consumption current exceeds 500 mA is used, there has been no other choice than providing an additional power supply circuit or an attachment.
However, to obtain a larger working space and a cost reduction, as well as high functionality of a device, it is further demanded, and desired particularly in recent years, to enable a variety of functions to be realized by receiving a supply of electric current that exceeds 500 mA without using an AC adaptor or an additional attachment.
In order to solve the above mentioned problem, the inventors of the present invention have filed a patent application for an invention relating to an electronic device capable of easily receiving electric current exceeding the maximum permissible current (500 mA) of one USB terminal using two or more USB interface signal cables. This is presented as a patent reference 1.
[Patent reference 1]
Japanese Laid-open Patent Application No. 2002-297269
In the above mentioned patent reference 1, the technical contents of a system are disclosed including firmware and a device driver in which a main unit of a USB device completely operates only when two sets of USB interfaces are connected. In detail, when a USB interface signal cable provided for a power supply is connected, the device acts as if a dummy USB device were connected on viewing from host equipment, such as a personal computer, for example. However, since such configuration forces a user to spend time and effort installing the device driver in the host equipment, usability deteriorates. There is also the case in which the installation of the software fails depending on a user, and it becomes impossible to provide a stable operation for the user. Moreover, the configuration of the device driver that is installed on the host equipment side and of the firmware that operates on the USB device side becomes complicated and therefore, not only development costs increase, but also bugs tend to be induced.
If simply a configuration of a second USB interface to which only a power supply cable is connected is provided in order to supplement a power supply of a conventional USB device, it is necessary to pay attention to a connection sequence of USB interface cables. In an apparatus configured in this manner, when a USB interface cable having a signal cable is connected first to a host apparatus, the device starts to operate since the signal cable is connected. Then, the USB device communicates with the host equipment in accordance with the USB protocol to inform the apparatus that a bus-power and high power device is connected, and finally it becomes possible to receive an electric current of 500 mA. However, since the power supply to the device is not yet sufficient, a shortage of a power supply capacity (electric current) with respect to the whole device occurs. As a result, operations of the device and of the host equipment become unstable. In other words, there is a possibility of causing a failure in the operation unless the two USB cables are connected simultaneously to the host equipment.